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The diagram below shows the few components needed to assemble a remote temperature alarm for your greenhouse. It utilizes "X-10" technology, which sends control signals over ordinary household electric wiring. I'm assuming that your greebhouse has electricity that originates from the same circuit the house is on, which offers the advantage of not requiring another long wire between the house and the greenhouse.
Most of the components can be purchased at your local Radio Shack, although it may be tough to find the Powerflash module. Another source for all of the X-10 components is www.smarthome.com.
The design pictured relies on a battery to power the alarm bell, but if you substitute a 110V sound device or flashing light, you can reduce the cost and simplify this further by using a lamp module, instead of the Powerflash unit, to activate your signal in the house. Likewise, you can use a heating thermostat in place of the temperature sensor to further reduce the cost.
Parts list from www.smarthome.com:
4060 X-10 Powerflash module ($23.95) -
2010 X-10 Universal Module ($19.95)*
7151 Slimline Temperature Alert Sensor ($43.95) - 1 required (may substitute a thermostat)
* You may substitute one 2000IBM X10 Lamp Module @ $8.99 if you want to use a 110V alarm device.
You'll also need a battery, some lamp wire, and bell, buzzer, or other alarm mechanism, available from Radio Shack.
If you really want to add more features, there are devices available that warn of water in your basement, a power outage, low humidity, etc., and can hook a charger to the battery to make sure it's fully charged.
Whatever you do, be sure to test the alarm monthly!
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